Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers comprising porous, electrically conductive sheet material. An electrocatalyst is disposed at each membrane/electrode layer interface to induce the desired electrochemical reaction. The location of the electrocatalyst generally defines the electrochemically active area of the MEA.
In typical fuel cells, the MEA is disposed between two electrically conductive separator plates or fluid flow field plates. Fluid flow field plates have at least one flow passage formed therein to direct the fuel and oxidant fluid streams to the respective electrode layers, namely the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors and provide support for the electrodes.
Two or more fuel cells can be connected together, generally in series, but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack.
The stack typically includes inlet ports and manifolds for directing the fuel and the oxidant to the anode and cathode flow field passages respectively. The stack often also includes an inlet port and manifold for directing a coolant fluid to interior passages within the stack to absorb heat generated by the exothermic reaction in the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, as well as an exhaust manifold and outlet port for the coolant exhaust stream exiting the stack.
The fuel fluid stream typically comprises hydrogen. For example, the fuel fluid stream may be substantially pure hydrogen or a gas comprising gaseous hydrogen such as a reformate stream. Alternatively, a liquid fuel stream such as aqueous methanol may be used. The oxidant fluid stream which is supplied to the cathode typically comprises oxygen, such as, for example, air or another dilute oxygen stream.
It is desirable to seal reactant fluid stream passages in fuel cell stack to prevent leaks or inter-mixing of the fuel and oxidant fluid streams. Fuel cell stacks typically employ fluid tight resilient seals, such as elastomeric gaskets between the separator plates and membranes. Such seals typically circumscribe the manifolds and the electrochemically active area. Sealing is effected by applying a compressive force to the resilient gasket seals.
Fuel cell stacks are compressed to enhance sealing and electrical contact between the surfaces of the separator plates and the MEAs, and sealing between adjacent fuel cell stack components. In conventional fuel cell stacks, the fuel cell stacks are typically compressed and maintained in their assembled state between a pair of end plates by one or more metal tie rods or tension members. The tie rods typically extend through holes formed in the stack end plates, and have associated nuts or other fastening means to secure them in the stack assembly. The tie rods may be external, that is, not extending through the fuel cell plates and MEAs, however, external tie rods can add significantly to the stack weight and volume. It is generally preferable to use one or more internal tie rods which extend between the stack end plates through openings in the fuel cell plates and MEAs as described in U.S. Pat. No. 5,484,666. Typically resilient members are utilized to cooperate with the tie rods and end plates to urge the two end plates towards each other to compress the fuel cell stack.
The resilient members accommodate changes in stack length caused by, for example, thermal or pressure induced expansion and contraction, and/or deformation. That is, the resilient member expands to maintain a compressive load on the fuel cell assemblies if the thickness of the fuel cell assemblies shrinks. The resilient member may also compress to accommodate increases in the thickness of the fuel cell assemblies. Preferably, the resilient member is selected to provide a substantially uniform compressive force to the fuel cell assemblies, within an anticipated expansion and contraction limits for an operating fuel cell. The resilient member may comprise mechanical springs, or a hydraulic or pneumatic piston, or spring plates, or pressure pads, or other resilient compressive devices or mechanisms. For example, one or more spring plates may be layered in the stack. The resilient member cooperates with the tension member to urge the end plates toward each other, thereby applying a compressive load to the fuel cell assemblies and a tensile load to the tension member.
Conventional tension members typically have a mechanism for receiving a tensile load which is transferred to the tension member from the compression assembly. For example, the mechanism may be a protrusion or flange which typically bears against the resilient member or end plate to essentially prevent the end plates from moving apart. Conventionally, the ends of the tension member are threaded to receive a nut or other fastener which provides a bearing surface for receiving a tensile load. An advantage of a threaded connection is that the nut may be tightened to precisely adjust the tensile load applied to the tension member. However, there are several disadvantages of threaded connections, including the susceptibility of threaded ends to damage, the time required for assembly, and the production costs for high speed manufacturing processes. Threaded ends may be damaged, for example, by mishandling or by being stripped by the nuts during assembly (e.g. from misalignment or over-tightening). During assembly, to prevent the nuts from being over-tightened it may be necessary to monitor or limit the torque applied to the nuts.
A particular disadvantage with metallic tension members, and especially internal tension members, is that they need to be electrically insulated from the fuel cells in the stack to prevent the tension members from causing a short circuit. Metallic tension members are typically used with an electrically insulating sleeve. In addition to such sleeves being an additional component, a problem with sleeves is that repeated temperature cycles, the humid environment within the fuel cell assemblies, or other characteristics of the operating environment within the fuel cell stack may cause a sleeve to develop cracks or holes.
Still another disadvantage of metallic tension members is that they are also a potential source of metal ion contamination which can adversely affect the membrane electrolyte and/or electrocatalyst in the MEAs. For example, contamination may cause the electrocatalyst to become poisoned. Corrective action for electrocatalyst poisoning typically requires the fuel cell to be shut down. For electrocatalyst which is severely poisoned, it may be necessary to dismantle the fuel cell stack and replace the MEAs and the components which caused the contamination. Furthermore, metal ion contamination may be particularly extensive if the source of the contamination is internal tension members which extend through the interior of the fuel cell stack. Such tension members typically extend through the reactant fluid manifolds. Thus metal ions originating from a faulty metallic tension members can be transported to all of the interior fluid passages which are fluidly connected to the reactant manifold, including fluid passages leading to the electrocatalyst layer.
Accordingly, there is a need for an improved compression assembly which mitigates some or all of the aforementioned disadvantages which are associated with conventional compression assemblies which employ electrically conductive metallic tension members.